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Humans are constantly re-shaping the environment by building sprawling cities, but a new study demonstrates that urban environments are also altering the way life itself evolves – and it’s happening all around the world.

Researchers, led by evolutionary biologists at the University of Toronto Mississauga, analyzed data about the white clover plant that was collected by 287 scientists in 160 cities across 26 countries, from Toronto and Tokyo to Melbourne and Munich. They found the clearest evidence yet that humans in general, and cities specifically, are a dominant force driving the evolution of life globally – with white clover frequently evolving in direct response to environmental changes taking place in urban settings. 

The results of the  were .

“We’ve long known that we’ve changed cities in pretty profound ways and we’ve dramatically altered the environment and ecosystems,” says study co-lead James Santangelo, a PhD student in biology at �鶹ֱ��app Mississauga. “But we just showed [the reverse] happens, often in similar ways, on a global scale.”

The GLUE study illustrates that the environmental conditions in cities tend to be more similar to each other than to nearby rural habitats. In that sense, downtown Toronto is more comparable to downtown Tokyo than it is to surrounding farmland and forests outside of the city.

Not only were researchers able to observe global adaptation to cities, they identified the genetic basis of that adaptation and the environmental drivers of evolution. White clover produces hydrogen cyanide as both a defense mechanism against herbivores and to increase its tolerance to water stress, and GLUE found that clover growing in cities typically produce less of it than clover in neighbouring rural areas due to repeated adaptation to urban environments.

The changes in the presence of herbivores and water stress in cities are pushing white clover to adapt differently than their rural counterparts – a finding that holds true for cities across various climates and which holds implications that reach far beyond the humble clover plant.

James Santangelo, a PhD student in biology at �鶹ֱ��app Mississauga, co-led the study, which analyzed data about the white clover plant that was collected by 287 scientists in 160 cities across 26 countries (photo by Nick Iwanyshyn)

“This study is a model to understand how humans change the evolution of life around us,” says Rob Ness, an assistant professor of biology at �鶹ֱ��app Mississauga who co-led the project with Santangelo and Professor Marc Johnson. “Cities are where people live and this is the most compelling evidence we have that we are altering the evolution of life in them.

“Beyond ecologists and evolutionary biologists, this is going to be important for society.” 

GLUE examined white clover because it is one of the few organisms present in almost every city on Earth, providing a tool to understand how urban environments influence evolution.

Johnson says now that we know humans are driving evolution in cities across the planet, the information can be used to start developing strategies to better conserve rare species and allow them to adapt to urban environments. It can also help us better understand how to prevent unwanted pests and diseases from adapting to human environments, he says.

For GLUE, this study is just the beginning. Using the same techniques, collaborators collected more than 110,000 clover samples from 160 cities and nearby rural areas and have sequenced more than 2,500 clover genomes, creating a massive dataset that will be studied for years to come.

The unprecedented global collaboration began with a single tweet.

“Nearly everyone we asked to collaborate said yes – and that was kind of remarkable, because we were asking people to take on a lot of work,” says Johnson, who co-ordinated the more than 280 other researchers who participated in the study. “Our collaborators recognized the importance of this project. There has never been a field study of evolution of this scale, or a global study of how urbanization influences evolution.

"It would have been impossible to do this without our global set of collaborators.”

Johnson calls the project a model for inclusive science since team was equally split between women and men, and included not just established researchers, but also students at all levels and from all inhabited continents around the world.